Interaction of GTP‐binding proteins with calmodulin

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Abstract
Two GTP‐binding proteins (Gi and Go), which were the substrates for islet‐activating protein, pertussis toxin, were purified from bovine cerebral cortical membranes. Both Gi and Go completely inhibited calmodulin‐stimulated cyclic nucleotide phosphodiesterase activity. The same concentrations of these proteins, however, had no appreciable effect on the basal phosphodiesterase activity. The isolated Giα and βγ subunits of GTP‐binding proteins were potent inhibitors of the calmodulin‐stimulated phosphodiesterase activity, but Goα was very weak. Therefore, the βγ subunits were likely to be the major active molecules in the brain membranes. GTP‐binding proteins were shown to bind directly to calmodulin in a Ca2+‐dependent manner by a gel permeation binding experiment.